Subject: The theory of evolution INTRODUCTION Evolution is one of the most powerful theori

From: Chris Colby
Subject: The theory of evolution
INTRODUCTION
Evolution is one of the most powerful theories science has
ever known. For a variety of reasons, however, it is also one of
the most misunderstood. One common misunderstanding is that
the phrase "survival of the fittest" summarizes evolutionary theory.
In fact, it does not. The phrase is both incomplete and misleading.
The notions that evolution represents progress and, that organisms
can be arranged on an evolutionary ladder from bacteria to man, are
two other common misunderstandings.
This post is an outline of the basics of evolutionary
theory. It is intended to be a brief overview of the concepts and
mechanisms of evolution. Creationist arguments are not addressed
directly here; nor is a "laundry list" of reasons to believe in
evolution provided.
(I'm still not done with this post -- insert
standard excuse. As always, questions/comments are appreciated.
Be aware that there are typos. All the info here is the standard
dogma in the field, exceptions exist for some of my statements.
Also, some of the points made here are not universally agreed
upon by biologists. Evolution wouldn't be an exciting and in-
teresting field of science if they were. The majority of the
post is, I believe, noncontroversial to biologists in the field.)
WHAT IS EVOLUTION?
Evolution is a change in the gene pool of a population over
time. The gene pool is the set of all genes in a species or population.
The English moth, _Biston__Bistularia_, is a frequently cited example
of observed evolution. In this moth, rare black variants spread through
the population as a result of their habitat becoming darkened by soot
from factories. Birds could see the lighter colored moths more readily
and ate more of them.
Thus, the moth population changed from mostly light colored
moths to mostly dark colored moths. Since their color was determined
by a single gene, the change in moth color represented a change in the
gene pool. This change, by definition, was evolution.
Many creationists, when confronted with this example,
say "You started with moths and ended with moths... where's the
evolution?" The kind of evolution documented above is termed by
some "microevolution", while larger changes (taking more time)
are termed "macroevolution". Some biologists feel the mechanisms
of macroevolution are different from those of microevolutionary
change. Others, including myself, feel the distiction between the
two is arbitrary. Macroevolution is cummulative microevolution.
In any case, evolution is defined as a change in the
gene pool. Later in this post I will discuss macroevolution as
well as microevolution. For the sake of brevity I will use the
terms as if it is useful to draw a distiction between them.
WHAT ISN'T EVOLUTION?
For many people evolution is equated with morphological
change, i.e. organisms changing shape or size over time. An example
would be a dinosaur species slowly turning into a bird species.
It is important to note that evolution is often accompanied by
morphological change, but this need not be the case. Evolution can
occur without morphological change; and morphological change can
occur without evolution. For instance, humans are larger now than
in the past few hundred years, but this is not an evolutionary
change. Better diet and medicine brought about this change,
so it is not an example of evolution. The gene pool did not
change -- only it's manifestation did.
An organism's morphology is determined by both its genes and
its environment. Morphological changes induced solely by changes in
environment do not count as evolution, because this change is not
heritable. In other words, the change is not passed on to the organisms
offspring. Most changes due to environment are fairly subtle (e.g. size
differences). Large scale morphological changes (such as dinosaur to bird)
are obviously due to genetic changes, and therefore are evolution.
HOW DOES EVOLUTION WORK?
If evolution is a change in the gene pool; what causes the
gene pool to change? Several mechanisms can bring about a change in
the gene pool, among them: natural selection, genetic drift, gene flow,
mutation and recombination. I will discuss these in more detail later.
It is important to understand the difference between evolution (change
in the gene pool) and the mechanisms that bring about this change.
Bringing about a change in the gene pool assumes that
there is genetic variation in the population to begin with. Genetic
variation is "grist for the evolutionary mill". For example, if
there were no dark moths, the population could not have evolved from
mostly light colored to mostly dark colored. In order for continuing
evolution there must be mechanisms to both increase genetic variation
(e.g. mutation) and decrease it (e.g. natural selection, genetic drift).
HOW IS GENETIC VARIATION DESCRIBED?
Genetic variation has two components: allelic diversity
and non-random associations of alleles. Alleles are different
versions of the same gene at a given locus. For example,
at one eye color locus (locus means location) humans can have
the blue allele or the brown allele (or perhaps a green allele).
Most organisms, including humans, are diploid. This means they contain
two alleles for every gene at every locus. If the two alleles are
the same type (for instance both blue eye alleles) the individual
would be termed "homozygous" for that locus. An individual with two
different alleles is called "heterozygous".
Allelic diversity is simply the number of alleles at
each locus scaled by their frequency in the gene pool. At any
given locus there can be many different alleles in the gene pool.
It is important to realize that there can be more alleles in the
gene pool (at a given locus) than any single organism can possess.
Linkage disequilibrium is a measure of association of
alleles in the gene pool. If each gene assorted entirely
independently, the gene pool would be at linkage equilibrium.
However, if some alleles were often found togethor in organisms
(ie. did not assort randomly) these genes would be in linkage
disequilibrium. Linkage disequilibrium can be a result of physical
proximity of the genes or maintained by natural selection if some
combinations of alleles work better as a team.
HOW MUCH GENETIC VARIATION IS THERE?
Considerable variation has been detected in natural
populations. At about 70% of gene loci, there is more than
one allele present in the gene pool. Any given individual is
likely to be heterozygous at 30% of it's loci. Most loci
have been found to be assorting independently (i.e. they are
at linkage equilibrium). In most populations, there are
enough loci and enough different alleles that every individual
(barring monozygotic twins) has a unique combination of alleles.
MECHANISMS THAT DECREASE GENETIC DIVERSITY
MECHANISMS OF EVOLUTION: NATURAL SELECTION
Natural selection is held to be the most important
mechanism as far as adaptive evolution is concerned; it
is defined as differential reproductive success. Selection is
not a force in the sense that gravity or magnetism is. However,
biologists often, for the sake of brevity, refer to it that way.
Selection is not a guided or cognizant entity; it is simply an effect.
Some organisms have genes that enable them to reproduce more efficiently
than others of their species. Organisms with these genes, therefore
eventually replace the others of their species without these genes.
If environmental conditions change, new traits (new
combinations of alleles) will be selected for. Natural selection is a
mechanism that allows organisms to adapt to their current environment
only; it does not have any foresight. Traits or structures do
not evolve for future utility. The organism must be, to some
degree, adapted to it's environment at each stage of it's evolution.
Of course, this raises the question; how do complex traits
evolve? If half a wing is no good for flying, how did wings evolve?
Half a wing may be no good for flying, but it may be useful in other
ways. Feathers are thought to have evolved as insulation (ever
worn a down jacket?) and/or as a way to trap insects. Later,
proto-birds may have learned to glide when leaping from tree to tree.
Eventually, the feathers that originally served as insulation
now became co-opted for use in flight.
A traits current utility is not always indicative of it's
past utility. It can evolve for one purpose, and be used later for
another. A trait evolved for it's current utility is called an
adaptation; a trait evolved for another utility than it's current
use is termed an exaptation.
Natural selection works at the level of the individual.
The example I gave earlier was an example of evolution via natural
selection. Dark colored moths had higher reproductive success
because light colored moths suffered a higher predation rate.
The decline of light colored moths was caused by light colored
individuals being removed from the gene pool (selected against).
It is the individual organism that either reproduces or fails
to reproduce. Individual genes are not the unit of selection
(because their success depends on the organisms other genes as
well); niether are groups of organisms a unit of selection.
The individual organism is what reproduces or fails to
reproduce. It competes primarily with others of it own species for
it's reproductive success. For this reason, organisms do not perform
any behaviours that are for the good of the species. Natural
selection favors selfish behavior because any truly altruistic
act increases the recipient's reproductive success while lowering
the donors. Altruists would quickly disappear from a population
as the non-altruists would get the benefits, but not pay the cost,
of being an altruist.
Of course, many observable behaviors appear, at first
glance, to be altruistic in nature. Biologists, however,
can demonstrate (in the cases they have studied) that these
behaviors are only apparently altruistic. Cooperating with or
helping other organisms is often the most selfish strategy for
an animal.
Of all the mechanisms of evolution, natural selection
has the potential to change gene frequencies the fastest. It usually
acts to keep gene frequncies constant, however. This led a
famous evolutionist, George Williams, to say "Evolution proceeds
in spite of natural selection".
MECHANISMS OF EVOLUTION: GENETIC DRIFT
Another important mechanism of evolution is genetic drift.
Drift is a binomial sampling error of the gene pool. What this
means is, the genes that form the next generation are a sample
of the genes in the current generation.
Organisms produce more gametes than are needed. Females
produce many more eggs than are ever fertilized and males produce
billions of sperm that never fertilize an egg. The genes in this
sample of gametes are likely to be slightly different than the genes
in the parental gene pool due solely to chance. Drift is a rather
abstract concept to some; I will try to explain it via a somewhat
simple analogy.
Imagine you had a swimming pool full of one million marbles
(this will represent the parental gene pool), half are red and
half are blue. If you repeatedly picked ten marbles out, do you think
you would get five red and five blue every time (assume you replaced
your sample to the pool each time)? If you picked one hundred
marbles out, do you think you would get fifty red and fifty blue out
every time? In both cases the answer is no, some times the
frequency of red marbles in the sample would deviate from 0.50.
In the case of the 100 marble sample, the frequency of red
marbles would deviate much less, however.
If, after picking out ten or one hundred marbles, you refilled
the pool with marbles at the frequency of that sample and repeated
the process over and over; what do you think would happen? What
would happen is that the frequency of red to blue would fluctuate
over time. Eventually, there would be only one color marble left
in the pool. This is roughly analogous to how genetic drift works.
Both natural selection and genetic drift decrease genetic
variation. If they were the only mechanisms of evolution, populations
would eventually become genetically homogenous and further evolution
would be impossible. There are, however, mechanisms that replace
variation depleted by selection and drift. These are discussed below.
MECHANISMS THAT INCREASE GENETIC DIVERSITY
MECHANISMS OF EVOLUTION: MUTATION
A mutation is a change in a gene. There are many kinds of
mutations. A point mutation is a mutation in which one "letter" of
DNA is changed to another. Lengths of DNA can also be deleted or
inserted in a gene; these are also mutations. Finally, genes or parts
of genes can become inverted or duplicated.
Mutation is a mechanism of evolution because it changes
allele frequencies very slightly. If an allele "A" mutates to
another allele "a", the frequency of "a" has increased from zero
to some small number (1/2N in a diploid population where N is the
effective population size). The allele "A" will also decrease
slightly in frequency. Evolution via mutation alone is very slow;
for the most part, mutation just supplies the raw material for
evolution -- genetic variation.
Most, but not all, mutations are neutral or slightly
deleterious. Because the genetic code is redundant and genes
contain sequences (introns) that do not code for anything, any
change in single nucleotide of DNA is not likely to have much of
an effect. Most mutations that produce any noticable phenotypic
effect are substantially deleterious. Natural selection quickly
"weeds out" these mutations from the gene pool.
Rarely, though, mutations are beneficial. A great example
of this occurred recently in the mosquito _Culex pipiens_. A gene
that helped the organism degrade insecticide became duplicated.
Progeny from the mosquito this mutation occurred in quickly swept
over vast geographic areas, because their increased tolerance to
insecticides made them able to leave more progeny, on average, than
non-mutated mosquitoes.
[expand - examples]
Mutations are random with respect to their adaptive significance.
[expand - preadaptive mutations - "directed"
mutagenesis - Lamarckian evolution]
MECHANISMS OF EVOLUTION: RECOMBINATION
recombination
inter- and intra- genic recomb
exon shuffling
MECHANISMS OF EVOLUTION: GENE FLOW
gene flow/ horizontal transfer
SPECIATION
Speciation is the process of a single species becoming
two or more distinct species. Many biologists feel speciation is
key to understanding evolution. These biologists believe
certain evolutionary phenomena apply only at speciation
and macroevolutionary change cannot occur without speciation.
Other biologists think major evolutionary change can occur
without speciation. Changes between lineages are only an extension
of the changes within each lineage. In general, paleontologists
fall into the former category and geneticists in the latter.
MODES OF SPECIATION
Biologists recognise two types of speciation: allopatric and
sympatric speciation. The two differ in geological distribution of
the populations in question.
Allopatric speciation is thought to be the most common form
of speciation. It occurs when a population is split into two (or
more) subdivisions that organisms cannot bridge. The two populations
are geographically isolated; organisms from subdivision A can
only breed with organisms from subdivision A and B organisms can
only breed with B organisms. Eventually, the two populations gene
pools change (both independently) until they could not interbreed even
if they were brought back togethor. In other words they have speciated.
Sympatric speciation occurs when two subpopulations become
reproductively isolated without first becoming geographically
isolated. Monophytophagous insects (insects that live on a
single host plant) provide a model for sympatric speciation.
If a group of insects switched host plants they would not breed
with other members of their species still living on their former
host plant. The two subpopulations could diverge and speciate.
Some biologists call sympatric speciation microallopatric speciation
to emphasize that the subpopulations are still physically separate
not at a geographic level, but on an ecological level.
Biologists know little about the genetic mechanisms of
speciation. Some think series of small changes in each subdivision
gradually lead to speciation; others think there may be a few key
genes that could change and confer reproductive isolation. (One
famous biologist thinks most speciation events are caused by
changes in internal symbionts. Most doubt this, however.)
Populations of organisms are very complicated. It is
likely that there are many ways speciation can occur. Thus, all of
the above ideas may be correct, each in different circumstances.
OBSERVED SPECIATIONS
It comes as a surprize to some to hear that speciation has
been observed.
[give plant and fly examples]
ARE WE STILL EVOLVING?
Yes, evolution is still occurring; all organisms continue
to adapt to their surroundings and "invent" new ways of better
competing with members of their own species. In addition, allele
frequencies are being changed by drift, mutation and gene flow
constantly.
MACROEVOLUTION VS. MICROEVOLUTION
evolution not linear progress/scale of nature
punctuated equilibrium
WHO STUDIES EVOLUTION AND HOW IS IT STUDIED?
[mention rough number of ev biologist, journals,
techniques used/different fields etc.]
SOME BOOKS ABOUT EVOLUTION
Evolutionary Biology, by Douglas Futuyma, 1986, Sinauer, Sunderland,
Mass